The Scandinavians will travel to the Iberian nation for the first leg on November 15, with the return duel scheduled for Stockholm four days later.
"Portugal are favourites but they are going to be two fantastic matches," Ibrahimovic told AFP after the draw was made in Zurich on Monday.
"We're playing against a good team with many good players. Playoff matches are like two finals - it's completely different from group matches.
"I think we have to reduce our errors and the team who makes the most will miss the World Cup," concluded the Paris Saint-Germain striker.
Portugal coach Paulo Bento, meanwhile, is wary of the threat posed by Ibrahimovic.
He said in statement: "We have two tough games against a side that, as well as having one of the best strikers in the world, plays well as a team.
"Sweden finished second in a group that contained Germany, one of the contenders to win the whole tournament.
"The key is to be prepared, and to be confident that we have a real chance of reaching the finals."
European World Cup Qualifying play-off draw results:
Portugal v Sweden
Ukraine v France
Greece v Romania
Iceland v Croatia